Japanese patent application JP 2013-145123 A discloses a wide-angle reflection optical system with coaxial illumination. The optical system has a camera for forming an image of an object to be inspected on an area sensor of the camera. A branch optical element is disposed rearward a lens, which is close to the object side to be inspected. The branch optical element is located on an optical axis of the optical system. A light source is provided for coaxial illumination of the inspection object. The illumination luminous flux from the light source enters the branch optical element from a direction so that it crosses the optical axis of the image capture optical system.
Japanese patent application JP 2011-106912 A discloses an imaging illumination device that uses coaxial illumination to capture a bright image with an element to be imaged. The illumination light irradiated from a light source is made incident to a half mirror via a diffusion plate. A portion of the light is reflected and illuminates a substrate. A portion of the light reflected by the substrate passes through the half mirror and is incident on the imaging element of an imaging device. The pattern image formed on the substrate is photographed. A reflecting member (mirror) that totally reflects the illumination light to the object to be illuminated is fixed to the outside of the portion of the half mirror, where the image of an imaging object (imaging area) passes through and the incident side of the illumination light. The illumination light from the light source is reflected by the half mirror and has the substrate that is reflected by this reflecting member irradiated. As a result, the substrate (object to be illuminated) can be illuminated brightly.